This invention concerns a method of and apparatus for measuring the electrophoretic mobility.
Charged particles in an electrolyte solution migrate toward an anode or a cathode in the solution when they are applied with an electric field, and such phenomenon, i.e., electrophoresis can be utilized for the understanding of some properties of the solute particles since the charged particles migrate at a velocity inherent to the particles under constant external conditions. The electrophoresis, together with the electroosmosis and the like, belong to interfacial electrokinetic phenomena and electrophoretic apparatus, for instance, observing the movement of charged colloidal particles are well-known in the field relevant to surface charged minute substances as the apparatus for measuring the interfacial electrokinetic phenomena. Such electrophoretic apparatus have recently attracted attention also in the immunological field as effective means for obtaining various immunological information from the electrokinetic phenomena of leucocytic immune-cells in bloods, for instance.
The electrophoretic apparatus generally comprises a cuvette (measuring cell), an electrode vessel having electrodes, a thermostatic bath, an electrical power source and an electrophoretic mobility detector. The electrophoretic apparatus can determine the properties and the charged states of particles which are dispersed in an electrolyte solution placed in the cuvette as a medium for the electrophoresis, by applying a DC voltage between the electrodes to generate an electric field and measuring the velocity of the charged particles that migrate in the cuvette in accordance with the electric field thus applied. Electrophoretic mobility of the particles is determined on the basis of the moving velocity (V) of the particle and the intensity of the electric field (E), and the electrophoretic mobility thus determined can be expressed by the moving velocity of each of the particles per intensity of the electric field, that is, .mu./sec per V/cm.
By the way, upon measurement of the electrophoretic mobility, electroosmotic effect of the medium caused by the electrical charging on the cuvette has to be taken into consideration. In usual electrophoretic apparatus, the cuvette is generally made of glass material in view of the optical property, accuracy in the fabrication, easy of washing and the like and, since the cuvette is charged negatively as will be apparent from the chemical structure of glass, the medium (electrolyte for electrophoresis) in the cuvette attains an electrical equilibrium at a high positive ion concentration near the wall of the vessel. Accordingly, when an electric field is applied for electrophoresis on both sides of the cuvette in such a state, electroosmotical medium flow occurs along the wall together with the electrophoretic migration of the particles. Thus, the electrophoretic mobility actually measured for the particles is the apparent electrophoretic mobility, that is, the mathematical sum of the true electrophoretic mobility and the flow rate of the medium caused by the electroosmosis. In view of the above, only the particles situated on a position where the electroosmotic flow velocity is zero, that is, on a theoretical stationary plane have been selected as the object to be measured in the prior method employed so far for determining the true electrophoretic mobility. However, since the possibility that the particles situate on the theoretical stationary plane is extremely low and only a restricted portion of the particles contained in the test specimen can been measured in the prior method, it worsens the measuring efficiency and brings about a difficulty in the statistical analysis of the test specimen. Further, it is not always certain whether the particles situated on the stationary plane are being measured or not as described later.